1. Field of the Invention
One or more embodiments of the present invention relate to audio coding, and more particularly, to surround audio coding for an encoding/decoding for multi-channel signals.
2. Description of the Related Art
Multi-channel audio coding can be classified into waveform multi-channel audio coding and parametric multi-channel audio coding. Waveform multi-channel audio coding can be classified into moving picture experts group (MPEG)-2 MC audio coding, AAC MC audio coding, and BSAC/AVS MC audio coding, where 5 channel signals are encoded and 5 channel signals are decoded. Parametric multi-channel audio coding includes MPEG surround coding, where the encoding generates 1 or 2 encoded channels from 6 or 8 multi-channels, and then the 6 or 8 multi-channels are decoded from the 1 or 2 encoded channels. Here, such 6 or 8 multi-channels are merely examples of such a multi-channel environment.
Generally, in such multi-channel audio coding, the number of channels to be output from a decoder is fixed by encoder. For example, in MPEG surround coding, an encoder may encode 6 or 8 multi-channel signals into the 1 or 2 encoded channels, and a decoder must decode the 1 or 2 encoded channels to 6 or 8 multi-channels, i.e., due to the staging of encoding of the multi-channel signals by the encoder all available channels are decoded in a similar reverse order staging before any particular channels are output. Thus, if the number of speakers to be used for reproduction and a channel configuration corresponding to positions of the speakers in the decoder are different from the number of channels configured in the encoder, sound quality is degraded during up-mixing in the decoder.
According to the MPEG surround specification, multi-channel signals can be encoded through a staging of down-mixing modules, which can sequentially down-mix the multi-channel signals ultimately to the one or two encoded channels. The one or two encoded channels can be decoded to the multi-channel signal through a similar staging (tree structure) of up-mixing modules. Here, for example, the up-mixing stages initially receive the encoded down-mixed signal(s) and up-mix the encoded down-mixed signal(s) to multi-channel signals of a Front Left (FL) channel, a Front Right (FR) channel, a Center (C) channel, a Low Frequency Enhancement (LFE) channel, a Back Left (BL) channel, and a Back Right (BR) channel, using combinations of 1-to-2 (OTT) up-mixing modules. Here, the up-mixing of the stages of OTT modules can be accomplished with spatial information (spatial cues) of Channel Level Differences (CLDs) and/or Inter-Channel Correlations (ICCs) generated by the encoder during the encoding of the multi-channel signals, with the CLD being information about an energy ratio or difference between predetermined channels in multi-channels, and with the ICC being information about correlation or coherence corresponding to a time/frequency tile of input signals. With respective CLDs and ICCs, each staged OTT can up-mix a single input signal to respective output signals through each staged OTT. See FIGS. 4-8 as examples of staged up-mixing tree structures according to embodiments of the present invention.
Thus, due to this requirement of the decoder having to have a particular staged structure mirroring the staging of the encoder, and due to the conventional ordering of down-mixing, it is difficult to selectively decode encoded channels based upon the number or speakers to be used for reproduction or a corresponding channel configuration corresponding to the positions of the speakers in the decoder.